1. Field of the Invention
The present invention relates to a porous film manufacturing method and a successive biaxial stretching apparatus for manufacturing a porous film. Particularly, the present invention relates to a porous film manufacturing method in which micropores are formed by stretching by use of phase separation in a binary system, and to the successive biaxial stretching apparatus for manufacturing the porous film.
2. Description of the Related Art
There is known a method for manufacturing a porous film having micropores, which is used for a battery separator, a hemodialysis membrane, an air-permeable film, a filter or the like. The method includes: thermally melting polyethylene (polyolefin) powder by using oil or liquid paraffin as a solvent; performing phase separation; and thereafter forming micropores by stretching (see Japanese Patent Application Laid-Open No. 2003-103626, for example).
A raw film includes oil or liquid paraffin as a solvent, and is thermally stretched. For these reasons, a large amount of oil is precipitated on the surfaces of the raw film. A general longitudinal stretching machine is not capable of stably stretching the raw film during its roll stretching (longitudinal stretching) operation, because the raw film slips on the surfaces of the respective rolls. In addition, bubbles occur to be involved in an interstice between each roll and the raw film, thereby causing unevenness in heating. This also makes it impossible to perform stable stretching.